1. Field of the Invention
The present invention relates generally to nuclear reactor fuel assemblies, and more particularly to nuclear reactor fuel assembly thimble tube caps which are operationally associated with the fuel assembly thimble tubes so as to provide the same service functions as the conventional fuel assembly thimble tube plugs without, however, exhibiting the operational disadvantages and impediments characteristic of the thimble tube plug system.
2. Description of the Prior Art
As is well known in the nuclear power plant art, the nuclear reactor core of a typical facility or plant may comprise, for example, at least one hundred or more fuel assemblies, and each fuel assembly, in turn, may comprise, for example, approximately several hundred fuel rods. As an example, a conventional fuel assembly may comprise what is known as a 17.times.17 array of fuel rods, which means that there are seventeen rows or columns of fuel rods, and each row or column contains seventeen fuel rods. In a similar manner, a multitude of control rods are adapted to be disposed within the fuel assemblies, and accordingly, a symmetrically arranged square array of thimble tubes, within which the control rods are capable of being disposed, are interspersed within the aforenoted array of fuel rods.
Depending upon the power requirements of the particular plant or facility at any particular time, the power output of the power plant or facility is accordingly controlled in the well-known conventional manner through means of the control rods and their disposition within the core fuel assemblies. In particular, not only is the depth to which the control rods are disposed within the fuel assemblies accordingly controlled, but in addition, the number of control rods within particular fuel assemblies is likewise controlled. Specifically, within a typical 17.times.17 array of fuel rods, provision is made for the accommodation of twenty-four control rods within such a fuel assembly. In accordance with the aforenoted power requirements and corresponding power output of the facility, however, the actual number of control rods employed or disposed within any particular fuel assembly will vary from zero to twenty-four. In those instances wherein control rods are not employed or disposed at specific locations within the fuel assembly, the thimble tubes must be effectively substantially closed in order to prevent any substantial amounts of debris present, for example, within the closed loop coolant system, from collecting within the thimble tubes or other portions of the reactor core. In addition, the volume flow rate of the water coolant through the thimble tubes must be appropriately controlled.
In order to achieve the foregoing operational objectives, thimble plug systems have been conventionally employed in connection with those thimble tubes which do not have control rods operatively associated therewith. With reference being made to FIG. 1 of the drawings, it is seen that each thimble tube plug system comprises a thimble tube 10 which fixedly interconnects the top and bottom nozzles of the fuel assembly with which the thimble tube 10 is operatively associated, it of course being understood that a multitude of thimble tubes 10, and not merely a single thimble tube 10, interconnects the top and bottom nozzles of each fuel assembly. In particular, the upper end of thimble tube 10 is inserted within a bore 12 defined within a top nozle adaptor plate 14 of the fuel assembly and is welded about its periphery to the upper surface of top nozzle adaptor plate 14 as shown at 16. It is noted that the lower end of each thimble tube 10 will also be connected to a bottom nozzle top plate by suitable conventional means, such as, for example, a shoulder bolt threadedly engaged within the thimble tube end plug, not shown.
As has been noted hereinabove, the thimble tubes 10 are adapted to house the reactor control rods, not shown, however, depending upon the particular power requirements of the particular reactor facility, the actual number of control rods employed or disposed within any particular fuel assembly may be less than the corresponding number of thimble tubes 10 within which the control rods are to be housed. In those instances, therefore, wherein control rods are not employed or disposed at specific locations within a particular fuel assembly, thimble plugs 18 have been disposed within the thimble tubes 10 in order to effectively substantially close-off or plug-up the tubes 10 so as to prevent the ingress of any debris, which may be present within the closed loop coolant system, from entering the thimble tubes or other portions of the reactor core and collecting therein.
As seen in FIG. 1, the thimble plugs 18 are substantially cylindrical in configuration and the upper ends of the thimble plugs 18 are provided with an upstanding stem portion 20, the diameter of which is substantially less than that of the main plug body portion 18. In turn, the uppermost portion of the thimble plug 18 is fabricated in the form of an upstanding threaded bolt portion 22 which has a diametrical extent which is less than that of the thimble plug stem portion 20. In this manner, an annular flange or shoulder portion 24 is defined between the thimble plug stem portion 20 and the threaded bolt portion 22. A holddown assembly base plate 26 is disposed in a vertically spaced relationship above the top nozzle adaptor plate 14, and a through-bore 28 is provided within the holddown assembly base plate 26 so as to permit the threaded bolt portion 22 of the thimble plug 18 to pass therethrough. In this manner, a nut 30 may be threadedly engaged with the threaded bolt portion 22 so as to fixedly mount the thimble plugs 18 upon the holddown assembly base plate 26, and it is noted that when the nut 30 is fully threaded and tightened upon thimble plug bolt portion 22, nut 30 engages the upper surface 32 of the holddown assembly base plate 26 while the shoulder portion 24 of the thimble plug 18 engages the undersurface 34 of holddown assembly base plate 26. Lastly, to prevent retrograde movement of the nut 30 relative the threaded bolt 22, and therefore threaded disengagement thereof, a lockwire 36 is inserted within a groove or slot, not shown, defined within the threaded bolt 22 and subsequently welded to the nut 30 as at 38.
While it may therefore be appreciated that the thimble plug systems have served to perform their various designated functions, the thimble plugs 18 are quite expensive to fabricate. In addition, in those instances wherein a substantial number of thimble plugs 18 are utilized in conjunction with a particular fuel assembly, during a refueling operation, when, for example, a particular fuel assembly is being moved from one location within the reactor core to another location within the reactor core, or alternatively, a new or fresh fuel assembly is being inserted within the core, when the holddown assembly base plate 26 is being vertically positioned or adjusted relative to the top nozzle adaptor plate 14 and its thimble tubes 10, it has often proven to be quite difficult to properly align all of the thimble plugs 18 with respect to the thimble tubes 10. Consequently, the efficiency of the refueling operations is often considerably hampered and substantially reduced, particularly when it is remembered that such refueling operations or fuel cycle reloads are all accomplished remotely and under immersed or underwater conditions. Still further, during normal reactor operations when, for example, elevational adjustments of the control rod assemblies are desired to be accomplished in accordance with facility power requirements, it being appreciated that the control rod assemblies are connected to the holddown assembly base plate 26 in a manner similar to that by which the thimble plugs 18 are secured thereto, or alternatively, during the initiation or commencement of refueling operations, difficulty has likewise been experienced in connection with the movement of the thimble plugs 18 within the thimble tubes 10, or in connection with the withdrawal of the thimble plugs 18 from the thimble tubes 10, respectively. This difficulty has apparently developed as a result of debris within the closed loop coolant often becoming wedged between the thimble plugs 18 and the thimble tubes 10, such thereby interfering with the relative movement that may be sought to be accomplished between the thimble plugs 18 and the thimble tubes 10 during the aforenoted operational modes. It is to be noted that in accordance with conventional thimble plug systems, a small annular space 40 is defined between the thimble plugs 18 and the thimble tubes 10 in order to accurately control the volume flow rate of the closed loop coolant system through the thimble tubes, yet despite the provision of such an annular space 40, the aforenoted operational difficulties nevertheless prevail.
Accordingly, it is an object of the present invention to provide new and improved means for closing-off nuclear reactor fuel assembly thimble tubes within which control rods are not employed.
Another object of the present invention is to provide new and improved means for closing-off nuclear reactor fuel assembly thimble tubes within which control rods are not being employed which overcomes the various operational difficulties associated with conventional thimble plug systems.
Yet another object of the present invention is to provide new and improved means for closing-off nuclear reactor fuel assembly thimble tubes within which control rods are not being employed which satisfactorily achieves the operative objectives of conventional thimble plugs without exhibiting the operational disadvantages thereof.
Still another object of the present invention is to provide new and improved means for closing-off nuclear reactor fuel assembly thimble tubes within which control rods are not being employed which are substantially more economical to fabricate than conventional thimble plugs.
Yet still another object of the present invention is to provide new and improved means for closing-off nuclear reactor fuel assembly thimble tubes within which control rods are not being employed which will satisfactorily prevent the ingress of debris, which may be present within the closed loop coolant system, into the thimble tubes and the reactor core.
Still yet another object of the present invention is to provide new and improved means for closing-off nuclear reactor fuel assembly thimble tubes within which control rods are not being employed which will satisfactorily regulate the volume flow rate of the closed loop coolant through the thimble tubes in accordance with conventional closed loop coolant volume flow rate parameters.
A further object of the present invention is to provide new and improved means for closing-off nuclear reactor fuel assembly thimble tubes within which control rods are not being employed which do not present any alignment problems as was characteristic of conventional thimble plug systems.
A yet further object of the present invention is to provide new and improved means for closing-off nuclear reactor fuel assembly thimble tubes within which control rods are not being employed which do not present any impediment to the normal elevational adjustment of those control rods which may be employed within other fuel assembly thimble tubes.
A still further object of the present invention is to provide new and improved means for closing-off nuclear reactor fuel assembly thimble tubes within which control rods are not being employed which do not interfere with the conventional means for attaching the thimble tubes to the fuel assembly top nozzle adaptor plate.
A yet still further object of the present invention is to provide new and improved means for closing-off nuclear reactor fuel assembly thimble tubes within which control rods are not being employed which are quite simple to attach to the holddown assembly base plate.
A still yet further object of the present invention is to provide new and improved means for closing-off nuclear reactor fuel assembly thimble tubes within which control rods are not being employed which will eliminate the collection of debris within the thimble tubes as was characteristic of conventional thimble plug systems.